Formation testers and related sampling procedures for acquiring conventional oil samples from underground formations have been described in U.S. Pat. Nos. 4,860,581 and 4,936,139, amongst others. Example sampling procedures may include the use of sampling probes of various geometries and/or packer assemblies to fluidly connect the formation tester to the formation and extract fluid from the formation. Within the formation tester, flow-lines usually convey the fluid extracted from the formation through fluid analyzers, and eventually to one or more of a plurality of sample storage vessels that may be located several meters away from the point of entry (e.g. a sampling port) of the formation fluid into the formation tester. Typically, the diameter of the flow-lines may be on the order of 10 mm. Thus, the volume of an average 10 m flow-line between the point of entry of the formation fluid and a sample storage vessel may be approximately 800 cm3.
During sampling operations, the fluid initially present in the flow-lines is pumped out of the testing tool into the wellbore, and is progressively replaced by formation fluid extracted from the formation. In the cases when conventional oil (i.e. oil relatively mobile in the formation) is sampled, the flow-line volume is small compared with the volume of fluid that is usually extracted from the formation during a sampling operation. Indeed, it is not unusual to pump a volume on the order of 10,000 cm3 during the sampling operation, which is more than 10 times the flow-line volume mentioned above. Thus, the flow-line volume in the formation tester has usually a negligible impact on the sampling procedure. However, in the cases when heavy oil or bitumen, (i.e. hydrocarbon that may not be mobile at reservoir conditions) is sampled, it may be difficult to mobilize and extract a volume of formation fluid corresponding to the flow-line volume in addition to the volume of the fluid to be captured in a vessel of the formation tester.
For example, mobilizing the heavy oil and bitumen may be achieved by increasing the temperature of the formation near a sampling port of the formation tester. It should be appreciated that the thermal diffusivity of formations is many orders of magnitude lower than the thermal diffusivity of, for example, metals. Thus, the time required for the thermal wave to penetrate the formation sufficiently far into the reservoir to permit the temperature of an adequate volume of fluid to be increased and/or an adequate volume of fluid to be mobilized may be long. In particular, when using a resistive heating element positioned on the bore-hole wall, mobilizing about 1,000 cm3 of fluid close to a sampling probe while minimizing the thermal degradation of the hydrocarbon may require the formation to be heated for about two days. If mobilizing an additional volume of 1,000 cm3 is desired, then on the order of one more day may be required.